The present invention relates to method and apparatus for fine movement of a specimen stage of an electron microscope and more particularly to method and apparatus for fine movement of an electron microscope specimen stage in which the moving amount of the specimen stage is controlled in accordance with the enlargement magnification set for the electron microscope.
In conventional apparatus for specimen fine movement of the electron microscope, for the sake of moving a specimen, a drive handle of a drive mechanism having gears in combination is rotated by the operator so that the resulting torque is transmitted to a specimen stage through the drive mechanism to move the same.
With the conventional apparatus for specimen fine movement, the relation between the rotational amount of drive handle and the moving distance of specimen remains unchanged even when the enlargement magnification of the electron microscope changes and under the condition of the enlargement magnification being high, the moving amount of an enlarged image corresponding to the rotational amount of the drive handle becomes excessive, making it difficult to achieve, fine adjustments.
If the transmission ratio (gear ratio) of the drive mechanism is changed with a view of reducing the moving distance of the specimen corresponding to the rotational amount of the drive handle, the moving amount of an enlarged image corresponding to the rotational amount of the drive handle is unduly reduced under the condition of the enlargement magnification being low, raising a problem that to meet desirability for large movement of the enlarged image, the rotational amount of the drive handle must be increased unduly.
To solve the above problems, JP-A-55-46163 proposes an apparatus wherein the transmission ratio of the drive mechanism is changed in accordance with values of the enlargement magnification of the electron microscope to ensure that for individual different values of the enlargement magnification, the relation between the rotational amount of a motor corresponding to the aforementioned drive handle and the moving distance of an enlarged image on the phosphor screen can be substantially constant.
With the proposed apparatus which adjusts the relation between the rotational amount of motor and the moving distance of enlarged image by changing the transmission ratio of the drive mechanism, many gear ratio combinations are needed which comply with all steps, typically several of tens of steps for settable enlargement magnification, in order to accurately adjust the aforementioned relation for individual different values of the enlargement magnification, and realization of the gear ratio combinations is difficult from the standpoint of structural design of the mechanism. Therefore, the proposed apparatus can not succeed in keeping the relation between the rotational amount of motors and the moving distance accurately constant for individual values of the enlargement magnification.
In addition, the complicated drive mechanism will disadvantageously increase the size of the apparatus and degrade reliability.
According to another prior art disclosed in JP-B-52-701, due to the fact that in order to inspect the entire area of a specimen by moving the specimen in the X and Y directions while observing an enlarged image within the view field, it is necessary to move the specimen stage intermittently in such a manner that overlapping between the preceding view field and the succeeding view field is minimized as much as possible and the succeeding view field is prevented from going too far, the number of pulses (pulse number) supplied to a pulse motor to move the specimen stage by one stroke is controlled in accordance with individual values of magnification of the microscope.
According to still another prior art disclosed in JP-A-61-253756, the moving speed, V, of the specimen stage is controlled such that V*=M.multidot.V=constant stands, thereby making it possible to keep the moving speed V* of an image constant even when the enlargement magnification M of the microscope changes.